CHAPTER IV. CLUSTERS OF STARS AND NEBULE. The TELESCOPIO VIEW OF THE PLEIADES. 449. In surveying the concave of the heavens in a clear night, we observe here and there groups of stars, forming bright patches, as if drawn together by some cause other than casual distribution. Such are the Pleiades and Hyades in Taurus. These are called Clusters of Stars. luminous spot called the Bee Hive, in Cancer (visible to the naked eye), is somewhat similar, but less definite, and requires a moderate telescope to resolve it into stars. In the sword-handle of Perseus is another such spot or cluster, which is also visible to the naked eye, but which requires a rather better telescope to resolve it into distinct stars. When fairly in view, however, it is one of the most splendid and magnificent spectacles upon which the eye can rest. "O what a confluence of ethereal fires, From worlds unnumber'd down the steep of heaven, 450. Many of these faint and compact clusters have neen mistaken for comets, as through telescopes of mcd 449. Clusters? Specimens? 4.50. What mistake respecting? What like? How known that they are not mets ? erate power they appear like such. Messier has given a list of 103 objects of this sort, with which all who search. for comets ought to be familiar, to avoid being misled by their similarity of appearance. That they are not comets, is evident from their fixedness in the heavens, and from the fact, that when we come to examine them with instruments of great power, they are perceived to consist entirely of stars, crowded together so as to exhibit a defi nite outline, and to run up to a blaze of light in the cen ter, where their condensation is usually the greatest. 451. Some of these clusters are of an exceedingly rough figure, and convey the idea of a globular space filled full of stars, insulated in the heavens, and constituting in itself a family or society apart from the rest, and subject only to its own internal laws. It would be a vain effort to attempt to count the stars in one of these clusters. They are not to be reckoned by hundreds; and on a rough calculation, grounded on the apparent intervals between them at the borders, and the angular diameter ROUND CLUSTER IN CAPRICORN. of the whole group, it would appear that many clusters of this description must contain, at least, from ten to twenty thousand stars, compacted and wedged together in a round space, whose angular diameter does not exceed eight or ten minutes, or an area equal to a tenth part of that covered by the moon. 452. Some of these clusters have a very irregular out line. These are generally less rich in stars, and especially less condensed toward the center. They are also less definite in point of outline. In some of them, the stars are nearly all of a size; in others, extremely different. It is no uncommon thing to find a very red star, much 451. What said of the form of these clusters? Stars in each? Apparent diameter ? 452. What further respecting forms? Character of irregular clusters? brighter than the rest, occupying a conspicuous situation in them. Be RICH CLUSTER IN BERENICES' HAIR. 453. It is by no means improbable that the individual stars of these clusters are suns like our own, the centers of so many distinct systems, and that their mutual distances are equal to those which separate our sun from the nearest fixed stars. sides, the round figure of some of these groups seems to indicate the existence of some general bond of union, of the nature of an attractive force. This is one of the most gorgeous clusters in all the heavens. Sir John Herschel pronounced it the most magnificent object he had ever beheld. It is about 6' in diameter, and contains a countless throng of stars, that scarcely ever fail to elicit a burst of surprise and astonishment from the beholder! Who can gaze upon such a scene, and not for a time forget earth, in the rapt contempla tion of the distant glory? "There's not a scene to mortals given, That more divides the soul and clod, A similar cluster, though somewhat different in form, may be found between 6 and 7, In Hercules. This, too, is a most magnificent object. Under favorable circumstances, it may be seen with the naked eye; and by the aid of telescopes, it is easily resolved into myriads of stars. "It is, indeed, truly glorious," says Smyth, "and enlarges on the eye by studious gazing." "Perhaps," says Prof. Nichol," no one ever saw it, for the first time, through a telescope, without uttering a shout of wonder." NEBULE. 454. The term Nebula is applied to those clusters of stars that are so distant as to appear only like a faint cloud or haze of light. In this sense, some of the clusters heretofore described may be classed as nebulæ; and. indeed, it may be said of all the different kinds of nebu læ, that it is impossible to say where one species ends, and another begins. 453. What said of individual stars in clusters? Of round figure of some clusters? (What specimen in cut? What said of it? Angular diameter? Effect of seeing? Poetry? What other similar cluster? What said of it?) 454. What are Nebula? How differ from clusters? 455. Resolvable Nebula are those clusters, the light whose individual stars are blended together, when seer. through a common telescope; but which, when viewed through glasses of sufficient power, can be resolved into distinct stars. 456. Irresolvable Nebula are those nebulous spots which were formerly supposed to consist of vast fields of matter in a high state of rarefaction, and not of distinct stars. But it is doubtful whether any nebule exist which could not be resolved into stars, had we telescopes of sufficient power. "About the close of last year," says Dr. Scoresby, in 1846, "the Earl of Rosse succeeded in getting his great telescope into complete operation; and during the first month of his observations on fifty of the unresolvable nebulæ, he succeeded in ascertaining that 43 of them were already resolvable into masses of stars. Thus is confirmed the opinion, that we have only to increase the power of the instrument to resolve all the nebulæ into stars, and the grand nebular hypothesis of La Place into splendid astronomical dream." 457. Nebulæ of almost every conceivable shape may be found in the heavens. Some are roundothers elliptical. Some occur singly, while others are double, or seem to be connected together. The specimen here shown is in the Greyhound. The two nebulæ are elliptical, as shown, and are so united as to stand perpendicularly to each other. DOUBLE NELULE 458. Annular Nebula are those that exhibit the form of a ring. Of these, but few specimens are known. One of the most striking may be found about 6° below Mizar, 455. What are resolvable nebule? How when seen through powerful telescopes? 456. Irresolvable nebulæ? Are any nebulæ really irresolvable? Remarks from Dr. Scoresby ? 457. What further description of nebula? Specimen ? 458. What are annular nebula? Are they common? What specimen in cut? Describe it. the middle star in the tail of the Great Bear. It consists of a large and bright globular nebula, surrounded by a double ring, at a considerable distance from the globe; or rather a single ring divided through about two-fifths of its circumference, and having one portion turned up, as it were, out of the plane of the rest. A faint nebulous atinosphere, and a small round nebula near it, like a satellite, completes the figure. 459. Another very conspicuous nebula of this class may be found half-way between 6 and 7, in the Lyre, and may be seen with a telescope of moderate power. It is small, and particularly well defined, so as, in fact, to have much more the appearance of a flat oval solid ring, than of a nebula. The space within the ring is filled with a faint hazy light, uniformly spread over it, like a fine gauze stretched over a hoop. 460. "Planetary Nebulae," says Dr. Herschel, are very extraordinary objects. They have, as their name imports, exactly the appearance of planets-round or sightly oval discs-in some instances quite sharply terminated, in others a little hazy at the borders, and of a light exactly equable, or only a very little mottled, which, in some of them, approaches in vividness to that of the actual planets. Whatever be their nature, they must be of enormous magnitude." 461. Stellar Nebula, or Nebulous Stars, are such as present the appearance of a thin cloud, with a bright star in or near the center. They are round or oval 459. What other annular nebule? Describe. 460. Planetary nebulæ Describe. 461. Stellar nebula? Remarks of Professor Mitchol? |